1. Field of the Invention
The present invention generally relates to a security arrangement for and method of maintaining microprocessor-controlled electronic equipment normally operational until the occurrence of a disabling event, and thereupon for rendering the electronic equipment inoperative after the occurrence of the disabling event, as well as for maintaining the electronic equipment inoperative even after the disabling event has been terminated until a private code is entered to the microprocessor which controls the operation of the equipment.
2. Description of the Prior Art
There have been various proposals as to how to secure electronic equipment against theft. In the case of electronic equipment of the type controlled by a microprocessor manually accessed by a keyboard, it has been proposed, for example, in U.S. Pat. No. 4,103,289, in connection with an electronic calculator, to provide an electronic lock within the calculator which permits operation only after the operator has correctly operated a predetermined sequence of keys which constitutes the code. Prior to using the calculator for its normal calculating function, the code must be entered via the keyboard. The code is built into the memory of the microprocessor and is part of the internal silicon structure of the microprocessor chip. The built-in code is the same for each microprocessor in a production run, and is mass-produced along with the rest of the microprocessor.
Although generally satisfactory for its intended purpose, the above-described electronic calculator lock has proven to be inefficient and inconvenient to use. Since the code is built into the microprocessor chip, each and every calculator has the same fixed access code. This means that the access code would quickly become publicly known. Also, the fixed access code cannot be changed, and certainly not by the user. At best, the only way to change the fixed code would be for the integrated circuit manufacturer to design and build a different microprocessor with its own fixed code. It would, of course, be prohibitively expensive for the manufacturer to design and build a different custom-made microprocessor for each and every piece of electronic equipment to be protected.
Furthermore, it is objectionable to many users of electronic equipment to be compelled to unlock the equipment each and every time the equipment is to be used. For example, if the electronic equipment is a device having a keyboard and is to be operated on an intermittent basis, a user may not wish to be bothered with repeatedly having to unlock the equipment by manually entering the code prior to each use.
Another recent proposal in connection with theft-proofing automobile radios was made by Nakamichi U.S.A. Corporation in its commercial brochure entitled "Nakamichi Mobile Sound System" for use with its mobile tuner/cassette deck Model TD-1200. This car radio uses a built-in microprocessor and a set of push-buttons which act as a manual keyboard. A separate PROM (Programmable Read Only Memory) is connected to the microprocessor. Each PROM is electronically programmed at the factory with a different access code for each radio produced. The owner is provided with a tag imprinted with his unique access code. In use, each time before the radio can be turned on, the access code must be entered via the radio push-buttons.
The Nakamichi radio is made theft-proof by what is essentially an electronic lock which, although generally satisfactory for its intended purpose, has not proven to be altogether reliable in preventing theft. For example, the Nakamichi lock can be compromised in at least two ways. The contents of the separate PROM of a stolen radio can be interrogated by readily available electronic equipment and the access code can be easily deduced. The stolen radio can then be re-sold since the new owner will be provided with the correct code.
A second technique of compromising the Nakamichi lock is even simpler. A single Nakamichi radio can be purchased through a valid retail outlet. The radio would then be opened and the separate PROM removed and its contents interrogated as before. Duplicate PROMs can then be manufactured in any quantity desired. The code they represent is known from the tag originally provided. Thus, any stolen Nakamichi radios can be re-sold by replacing the original PROM with the duplicate clone PROM and providing the new owner with the known code.
In addition, the Nakamichi lock is like every other lock in the sense that the access code must be entered prior to each use of the equipment. Many users may not wish to be bothered by being repeatedly compelled to unlock the equipment and, hence, are prone to leaving the equipment unlocked at all times, thereby defeating the theft-proofing function.
It must also be noted that the Nakamichi lock is provided with a fixed, unchangeable code which has been selected by the factory. If the user forgets the code, then the user must contact the factory for the code. It would be desirable if the private code could be selected by the user to be a code that is easily remembered by him, e.g. his telephone number or social security number. This would minimize the problem of a forgetful memory. It would further be desirable if the user could himself change the code from time to time to any other code in order to increase the system security.
Aside from the electronic locks described above, additional anti-theft locks are described in U.S. Pat. Nos. 2,843,843; 3,656,114 and 4,090,089. Each lock requires a code to be entered in a proper predetermined order prior to using the equipment normally locked thereby.
It has further been proposed in U.S. Pat. No. 4,284,983 to protect an electrical appliance from theft and unauthorized movement by the setting of an alarm when the appliance is both disconnected and moved. In U.S. Pat. No. 3,815,088 an alarm is set off when an unauthorized entry is made into a vehicle. These alarms can both be easily by-passed, in which case, the item being protected is once again usable for its normal purpose.